The present invention concerns radial tires and the reinforcement of the carcasses thereof.
It is known that such carcass reinforcement includes at least one layer of ply, hereinafter called a radial ply or radial carcass ply, prepared in the form of a semi-finished product of essentially an elastomeric matrix reinforced by cross-threads. These reinforcement threads, hereinafter called radial threads, are arranged parallel to one another and at regular intervals in an essentially radial direction, i.e., a direction making an angle of approximately 90.degree..+-.10.degree. with the tangent to the circumference of the tire.
Each radial ply normally takes the form of a continuous annular element formed during the fabrication of the carcass reinforcement on a drum by cutting widths or strips of rubberized tire fabric and then joining together the two edges of the fabric thus cut on the drum, by means of a joint also known as a weld splice, which enables those edges to be joined. The strips of tire fabric are usually cut between and parallel to two radial threads to avoid cutting them.
Two types of joints or splices are currently in use: the so-called butt splice obtained by abutment of the two edges without overlap, and the so-called overlapping splice, i.e., one involving overlap of the two edges. In the case of radial tire for touring vehicles whose carcass is reinforced with non-metallic threads, notably by textile threads, a overlapping splice is the more usual.
The present invention relates to an improvement to overlapping splices of the radial ply constituting the carcass reinforcement of radial tires.
As its name suggests, the overlapping splice technique involves covering one edge of the cut fabric with its other edge over a certain distance, and joining the two together by the application of pressure. After curing of the tire carcass, this type of splice shows great strength because of the considerable area of mutual adhesion between the overlapped portions.
However, this overlapping splice has disadvantages. Because of the double thickness of tire fabric, the density of the radial threads is in effect doubled in the overlap zone. The term "density of threads", is a term which denotes the number of threads per unit length of ply in a direction normal to the axis of the threads. Because of the presence of this double layer, and notably under the effect of the inflation pressure of the tire and therefore the tensioning of the radial carcass ply, each radial thread in the seam is stressed and elongated about half as much as a thread outside the overlapped zone. Moreover, and in a known way, if the radial threads are thermo-shrinking they will contract differently during the curing of the tire, depending on whether they are within or outside the overlap.
The above phenomena result in a discontinuity of the properties of the carcass ply in the region of the overlap, notably its rigidity, such that the tire produced is not uniform. This discontinuity and non-uniformity, which are very localized, may be particularly harmful for the appearance, the external aspect of the tire, because they will in general result in the formation of a more or less pronounced hollow in the sidewalls of the inflated tire. It is known that this depression or deformation of the sidewall, also described as an indentation, is the more marked the lower is the tensile modulus of the reinforcing thread used.
The above appearance defect, which is well known and particularly visible when using threads of polyester with a low tensile modulus, is sometimes prejudicial to the marketability of the tire. If the defect is deemed too pronounced, it may even prevent any sales of the manufactured tire, resulting in the eventual destruction thereof, a destruction all the more regrettable from the functional standpoint since the unattractive appearance is not associated with any compromise of safety at all. Moreover, it is known that this non-uniformity of the tire sidewalls can have an adverse effect on the rolling regularity of the tire.
Very many prior solutions have been proposed to attempt to overcome the above disadvantages.
Most of these solutions involve modifying the characteristics of the radial reinforcement threads, or their arrangement in the part of the overlap. Such solutions possess the major disadvantage of necessitating either the preparation of hybrid liner fabrics, i.e. nonhomogeneous ones (the use of natural radial threads with different properties, or the use of different thread densities in the overlapped zone) upstream of the tire fabrication operations themselves, or costly additional operations during the fabrication (removal of doubled threads, reduction of overlapping thickness by techniques involving the compression or uniformization of the overlap zone, addition of other strips of fabric or rubber, to name but a few known examples).
Other solutions, fewer in number, have been proposed that consist in selectively cutting some of the "doubled" radial threads in one or the other of the overlapping edges. Thus, only the threads in one of the two edges will "take part" in the tensioning of the carcass ply. These cuts generally consist of narrow incisions (without removal of material) or wide notches (with removal of material--the so-called "notching" technique) made in the fabrication fabrics, which are generally located at some point on the sidewall of the tire as far as the inside part of the bead (bead zone situated on the inside of the casing, or even under the bead ring, see for example the patents U.S. Pat. No. 4,466,473 or EP 0 117 137, and the patent applications published under numbers JP1986/83025, JP1986/94745, JP1986/218402).
In particular, U.S. Pat. No. 4,466,473 or the equivalent EP 0 117 137 describe a process for the fabrication of a radial carcass reinforcement which consists in making, in just one of the overlap edges, all along that edge and at regular intervals, a plurality of cuts perpendicular to the direction of the radial threads, of depth essentially the same as the overlap width, the said edge then being covered by the other edge to form the overlapping splice. By cutting the radial threads of one overlapping edge in this way, the tension is eliminated in the threads of that edge, while they are doubled in the threads of the other edge. Thus, the threads in the uncut edge support a stress essentially identical to that in the threads outside the overlapping zone, such that tension variations between the inside and the outside of the overlap are eliminated, the hollowing deformation is appreciably reduced, and the uniformity of the sidewall improved.
The above methods involving selective cutting of the radial threads, applied to just one of the two overlapping edges, whether in the sidewall or on the inside of the bead, are particularly delicate to carry out. On the one hand the cut must be deep enough to go through all the doubled threads, but on the other hand it is imperative, for safety reasons, that none of the threads outside the splice zone are cut in the sidewall or inside the bead zone, so that the radial continuity of the carcass reinforcement outside the overlap zone is entirely preserved. Such conditions entail frequent manual interventions or repetitions by an operator on the fabrication drum, and these methods have ultimately proved to be incompatible with the use of automatic fabrication machines operating at a high rate, which cannot guarantee the precision that is essential for the formation of such cuts and overlaps. These disadvantages, which are well known, are mentioned for example in patents or patent applications EP 0 239 160 or U.S. Pat. No. 4,810,317, EP 0 407 134 or U.S. Pat. No. 5,437,751.
In fact, the very many solutions proposed so far have proved costly or difficult to apply industrially, and none of them has been found completely satisfactory, notably from the standpoint of productivity, for suppressing or even reducing the problem of sidewall deformation in radial tires, especially those reinforced with polyester threads.
Thus, there is still a demand, indeed an increasing one, to improve the appearance of radial tire sidewalls.